For many years the art has been aware of the benefits to be gained by driving a display such as a cathode ray tube with signals derived from a computer, such as a digital computer. The computer can be arranged to provide signals in a variety of formats depending upon the particular application, and usually some type of graphics generator is employed so as to produce signals of the type required to drive the display. In the case of a cathode ray tube, for example, the signals required to drive the display include beam deflection signals and unblanking signals. Deflection signals determine beam position in, for example, an x-y or an r-.theta. coordinate system. The pattern defining signals derived from the computer can be in a variety of forms.
At one extreme, the signals provided by the computer can actually define the movement pattern of the beam and may be connected directly, or through a digital/analog converter to the beam deflection system. Such an arrangement, however, can needlessly tie up a computer in generating beam position signals preventing the computer from doing more useful operations. It has therefore been appreciated that a simpler device can be driven by coded signals and the simpler device, i.e., the graphics generator, can actually produce the beam position signals necessary to produce the pattern or pattern segment defined by the input coded signal. For example, a vector can be defined by three quantities, a beginning point, a length and an orientation or angle and a graphics generator can be employed to convert these three quantities into beam position signals to cause the beam to traverse the desired pattern. In addition, an unblanking signal is required to illuminate the vector or line by unblanking the cathode of the cathode ray tube at the time the beam reaches the initial position, maintaining the cathode in an unblanked condition during the time the beam traverses the desired vector, and then blanking the cathode after the beam has passed the end of the vector.
Graphics generators capable of substantially performing the foregoing functions are well known in the art. With the advent of such graphics generators, it became apparent that special attention was required to control the writing speed of the beam at a constant rate, otherwise, pattern segments written at a slow rate would be over-illuminated, and other pattern segments written at a faster rate would be under-illuminated.
The desired goal of constant writing speed can be achieved, in a beam deflection system operating in an x-y coordinate arrangement, by controlling the incremental beam displacement per unit time such that the square root of the sum of the resolved incremental component displacements squared is equal to a constant. Such an arrangement is shown, for example, in U.S. Pat. Nos. 3,576,461; 3,725,723; 3,761,765; 3,869,085, and 3,818,475. However, in each of these systems, the resolved incremental offsets are derived from the computer, and thus to at least this extent, the computer is tied up and prevented from performing other tasks.
It has also been recognized in the prior art that the computer can be freed from unnecessary operations if predetermined patterns are defined by selected codes, and a graphics generator employed to generate beam position and unblanking signals responsive to the codes to produce the desired pattern. For example, alpha numeric patterns can readily be defined by the computer in shorthand (or code) form if the graphics generator can be provided to respond to the shorthand (or code) form to produce the necessary beam deflection and unblanking signals to produce the pattern. Such a device is disclosed, for example, in U.S. Pat. No. 3,952,297. As disclosed in that patent, a character code selects a memory location in which is stored data definitive of a series of micro strokes which can be employed to produce the desired character on a CRT or the like. While the patent alleges that it discloses an arrangement having minimal data storage requirements, nevertheless, it requires storage of a plurality of micro strokes, that is, the x and y coordinates of the end of each micro stroke for each character. In addition, it is not at all apparent that the apparatus disclosed can be employed to generate anything but the preselected characters whose definition is stored. Furthermore, since the storage includes the resolved increments in x and y directions, it is not seen that the arrangement is adaptable to provide the character at any selected orientation.
It is therefore one object of the present invention to provide a graphics generator which is capable of accepting coded signals from, for example, a digital computer definitive of a desired pattern, in which each of the signals represents a different pattern segment, and which is capable of operating on those pattern segment defining signals to produce digital signals for controlling beam deflection and unblanking of a display such as a cathode ray tube display to produce the desired pattern segment. It is another object of the present invention to provide apparatus such as apparatus of the type mentioned in which the beam deflection varies at a constant rate so as to produce constant brightness displays, regardless of the pattern segment, the size thereof or its orientation.
It is yet another object of the present invention to provide a graphics generator which is capable of producing signals for deflecting the beam of a display such as a cathode ray tube to display patterns comprising pattern segments in the form of vectors, conics or characters. It is another object of the invention to provide a graphics generator which includes a storage device defining each character to be displayed which further includes further apparatus so that such character can be displayed at any desired position or orientation in the display.